Transition Radiation Detector


The Transition Radiation Detector, or TRD, is a specialized subdetector on AMS. Its specific goal is to identify particles at very high energies.

  • Why we need the TRD To many detectors, all extremely-high-energy particles look similar. Protons, electrons, pions, muons - if they have a kinetic energy of 200 billion electron volts, it's hard to make the niggling distinction between their masses (protons with 1,000 million electron volts (MeV) of mass, electrons with 0.5 MeV, pions and muons with about 100 MeV). They have about the same momentum, so the tracker can't distinguish them. They all travel at about the speed of light, so the TOF and the RICH cannot distinguish them. The TRD, though, is sensitive to the quantity "gamma"; E/m, the energy divided by the mass. This quantity is very different for electrons and protons, so the TRD can tell them apart! In particular, this is important for the dark-matter search, for which we need to look at anti-electrons (positrons). If we routinely mistake protons for positrons, we cannot perform this search.

  • How the detector works
    At high energies, the TRD sees a small difference between an electron and a proton: At high energies, the electron will emit X-rays while crossing the detector, and the proton will not. The X-rays are generated whenever the particle crosses an "interface" - in this case, the interface between a piece of plastic and a vacuum. Each particle passes through many hundreds of interfaces when it crosses a batt of plastic cloth or felt.

    The X-rays pass through the fabric, and they ionize a mixture of gases (Xenon, which is easy to ionize, and CO2, which regulates the size of the signal) in a tube with a high-voltage wire. The ionized gas experiences a little "cascade" - sort of the beginning of a spark - as the free electrons rush towards the high-voltage wire, and the result is a signal that can be read out at the end of the wire. Every particle will make a signal as it passes through, but we will look for the enhancement of the signal due to the extra transition-radiation X rays.

  • How it is built The TRD is a big octagonal object sitting on top of AMS. It consists of 20 layers of plastic fabric, alternating with 20 layers of these gas-filled x-ray detecting tubes. The tubes are very difficult to manufacture, especially since they have to be gas-tight; there are something like 10,000 separate pieces to be sealed together with glue, which must not leak or crack for three years! We have to keep the tubes full of Xenon/CO2, in spite of any leaks or contamination, so each tube is connected to a large gas handling system. (Click on the link for more about the gas system).
  • The gas system Click here to learn about how we prepare the Xenon-CO2 mixture.
    Click here to return to the AMS tour Silicon tracker superconducting magnet superconducting magnet time of Flight counters tme-of-flight coutners Ring-imaging cerenkov counter electromagnetic calorimeter vacuum case Transition radiation detector anti-coincidence counter Anti-coincidence counter electronics racks electronics crates thermal control system Unique support structure Unique support structure Unique support structure cryocoolers cryocoolers Xenon storage tank CO2 storage tank Circulation and monitoring system Valves Pressure sensors radiator radiator High-voltage wire tube full of gas X-ray particle passing through High-voltage wire tube ful of gas detector octagon Upper support plate support structure ("M-structure")